Palladium/nickel alloy coatings may be applied to conductive substrates for decorative and/or technological purposes, see the aforementioned copending applications. For example, such coatings are useful because they can be employed as a substitute for gold coatings and have an appearance similar to that of gold and corrosion resistance which can be significant.
As described in the aforementioned applications and in British Pat. No. 1,143,178, such coatings are generally deposited from a bath which is an aqueous solution of palladium and nickel. The palladium content of the bath is usually around 5 to 30 grams per liter and the nickel content is substantially 5 to 30 grams per liter as well, the bath containing sulfonic acid salts among other additives and the palladium/nickel ratio in the solution being selected so that the galvanically deposited or electroplated coating will contain 30 to 90% by weight palladium.
The resulting coating can be used, as noted, as a replacement for gold coatings since it has an appearance and decorative effect similar to that of gold and various properties, e.g. as a contact material for electrical contacts, which are also similar to those of gold. Thus, such coatings have an important role in electrotechnology.
The most important characteristic apart from high conductivity that a material thus suitable for use in electrotechnology must possess is a high resistance to corrosion of all types. In some cases the earlier palladium/nickel coatings did not have sufficient corrosion resistance.
In the electrodeposition of such coatings, moreover, it is known to add brighteners to the bath. Such brighteners have been aromatic sulfonic acids and their salts or other derivatives.
Typical of such brigteners are naphthalene sulfonic acid salts and aromatic sulfonamides such as the sodium salt of naphthalene-1,5-disulfonic acid, the sodium salt of naphthalene-1,3,6-trisulfonic acid, saccharin (o-sulfobenzoic acid imide) and p-toluenesulfonamide.
Reference may also be had in this connection to U.S. Pat. Nos. 4,010,084 and 4,102,755.
In the earlier palladium systems, there is occasionally a detrimental spontaneous salting out of palladium in the form of an insoluble salt, especially when the sodium salt of naphthalene-1,5-disulfonic acid is used, with a result that the coating has an unsatisfactory appearance and technological quality, especially when p-toluenesulfonamide is used as a brightener as well. Although the same problems do not also arise when the electrolyte contains the sodium salt of naphthalene-1,3,6-trisulfonic acid and/or saccharin as brighteners, sufficient corrosion resistance may be a problem in these cases.
Corrosion resistance can be conveniently measured, for the purposes of the present invention, by the immersion of test strips for 60 seconds at room temperature in a dilute acid solution consisting of equal parts of concentrated nitric acid and water.
When corrosion resistance is mentioned herein, therefore, such test conditions are employed.
German Patent No. 1,028,407 describes the use of specific brighteners for the galvanic deposition of bright nickel coatings, the brighteners being added to the bath in an amount of 0.1 to 1 gram per liter, preferably 0.5 grams per liter. The brightener compound, which has the same general formula as is given below, functions exclusively as a brightener, making no noticeable contribution to improvements in corrosion resistance. The brightening effect appears to be related to the urea group of this molecule which operates in a manner similar to earlier urea brighteners, the imine group of the molecule also having a brightening effect.
As far as we have been able to ascertain, such compounds have never been proposed as corrosion resistance promoters for palladium/nickel coatings or analogous deposits.